Research On Impairment Measurement And Fault Detection Technology For Few-Mode Fiber

With the global data traffic increasing explosively,the development of new transmission technologies is a hot topic for research and competition between academic and industry.The mode division multiplexing?MDM?technology based on few-mode fiber?FMF?,which can break through the non-linearity Shannon limit of traditional single-mode fiber?SMF?communication and multiply the capacity of optical fiber communication system by utilizing the orthogonality of spatial modes for spatial diversity multiplexing,is the most competitive expansion scheme to solve the“bandwidth crisis”of optical communication network in the future.However,the mode coupling?MC?,differential mode group delay?DMGD?and mode dependent loss?MDL?will deteriorate the transmission performance severely and thus limit the transmission distance.In addition,the transmission characteristics of each spatial mode are different,and there are interactions between different modes,which make the fault detection of FMF link more complicated than that of SMF.Therefore,it is an important work to research the impairments measurement and fault detection technology of FMF.It is of great significance for quantifying the relationship between impairments and parameters of FMF,optimizing the structure of FMF,providing theoretical basis for impairments compensation of MDM system,and improving the transmission performance of the system.In this dissertation,the impairments measurement and fault detection methods for FMF are investigated by theoretical derivation and experimental verification.We analyze the generation mechanism of different impairments and the falut characteristics of FMF.Based on the analysis above,we make a thorough study of the theory and methods for measuring MC,DMGD,MDL and fault detection.The main contributions and the innovative points of this dissertation are summarized as follows.1.Aiming at the problem of large measurement error of MC caused by the attenuation blind zone introduced by Fresnel reflection in the front end of FMF,a scheme of MC distribution measurement system based on two photon lanterns and a FMF circulator is proposed.Firstly,based on the MC theory of FMF Rayleigh backscattering amplitude analysis,the mathematical model of mode-coupled measurement is established.Then,the measurement performance of the proposed system scheme is verified by measuring the MC distribution of FMFs with different number of modes.The experimental results show that the proposed measurement system can effectively eliminate the Fresnel reflection and the attenuation blind area of the front end of the FMF by removing the free-space optical path,thus improving the measurement accuracy of the mode coupling to a certain extent.It can also measure the MC distribution of different types of FMFs.In addition,considering that the influence of weak Rayleigh backscattering signals of non-excitation modes on the measurement accuracy of MC,combined with the measurement method proposed above,a measurement method based on adaptive wavelet threshold denoising is proposed.This method improves the amplitude fluctuation of backscattering of each spatial mode,extracts the characteristic information of the mode attenuation,and then guarantees the steady-state distribution of the MC ratio curve.The experimental results show that this method can further improve the measurement accuracy of MC,as well as improve the dynamic range of the measurement system,which can satisfy the measurement of MC distribution for long-distance FMF links.2.Considering that the limitation of mode multiplexer crosstalk to the measurement accuracy of MDL,a measurement method based on adjustable mode power ratio is proposed.This method is mainly realized by combining cutback technology and impulse response measurement.The measurement performance of this method is verified by measuring the MDL of FMFs with different number of modes.The experimental results show that this method has strong robustness for low extinction ratio mode multiplexer,and improves the measurement accuracy of MDL to some extent.In addition,in order to achieve selective measurement of the DMGD between the required spatial modes and reduce the interaction between spatial modes with similar delay,an improved method based on mode selective excitation is proposed.The feasibility of this method is verified by experiments.Finally,considering the low efficiency of single parameter measurement and the high complexity of measurement system,a synchronous measurement method of MDL and DMGD based on analyzing the Raleigh backscattering amplitude of FMF is proposed.Firstly,the amplitude distribution of each spatial mode is measured,and then combining with the mathematical model of the mode characteristics parameters of FMF,the nonlinear equations are solved to realize the simultaneous measurement of two kinds of impairment parameters.The experimental results show that the proposed method can realize the simultaneous measurement of the two impairment parameters conveniently and accurately,as well as reduce the complexity and cost of the system.And it is also demonstrated that the proposed scheme has good scalability and can be scalable to measure FMFs with a large number of modes.3.Considering that the problem of the low sensitivity of low-order mode fault detection in FMF,a fault detection method based on high-mode with high detection sensitivity is proposed.Firstly,the coupling characteristics of different spatial modes in the fusion splice of FMF are verified and analyzed experimentally.It is proved that the high-order mode is easier to characterize the loss characteristics of the fault amplitude,and the detection sensitivity is higher than that of the low-order mode.Then,the fault detection performance of the scheme is verified by experiment under different fusing quality conditions.The experimental results show that the proposed method can effectively characterize the fault events with different magnitude loss,and then realize the comprehensive evaluation of the fault events of the FMF links.In addition,aiming at the problems of low signal-to-noise ratio of non-excitation mode backscattering signal,which leads to low detection accuracy of micro-fault events and slow system response speed,a fault detection method based on?₁ trend filtering is proposed in this dissertation.This method uses its characteristics of piecewise linear optimal estimation to estimate the trend of each mode backscattering curves,so as to characterize and locate the fault events of FMF links.The experimental results show that the proposd method can improve the sensitivity of fault detection of each spatial mode,and then realize the effective characterization of micro-fault events and high detection efficiency.In this dissertation,based on the research of the impairment mechanism of FMF and coupling loss characteristics of fusion splice in FMF links,we propose some new methods for the measurement of MC,DMGD and MDL in FMF and fault detection of FMF links,respectively.The proposed methods have advantages in improving the accuracy of impairments measurement and fault detection,as well as reducing the system complexity and cost,which will provide some reference for the further research on intelligent monitoring,impariment measurement and fault location for multi-mode,multi-scene,multi-parameter simultaneous measurement of FMF links.